Asphalt Concrete (AC) consists of approximately 95% aggregate, by weight. Of this 95%, about 6% is smaller than 0.075 mm in size (passing the #200 sieve and called fines).
The fines often contain mica. Mica is a formation of silicate minerals having perfect basal cleavage. It can be peeled apart in very thin sheets and is usually found in deposits of granite, quartz and other rock that is commonly used for aggregate. Mica has been shown to reduce the strength of asphalt concrete. This study evaluates the effects of mica on asphalt materials subjected to aging.
A total of five different concentrations of mica in fines are examined using X-ray diffraction (XRD) technology. These mica-fines are combined with two asphalt binders to make mastics. Mastics are aged at four different levels and examined with the Scanning Electron Microscope (SEM). Bending Beam Rheometer (BBR) and nanoindentation tests are conducted on mastics mixed with one of the binders and various concentrations of mica-fines aged at different levels.
In this study, SEM images of mastic are taken. During the mixing of the mastics, it is found that as the concentration of mica in the fines increases, so does the absorption of the binder. This is probably because mica has a flat surface that increases the surface area of the total aggregate. As the weight percentages of aggregate to binder is held exactly constant in the experiments conducted in this study, the mastics with lower concentrations of mica are found to be very rich while the higher concentrations are dry. SEM images show that cracks in broken mastic seem to follow uncoated mica flakes. The number of uncoated flakes increases only slightly with aging, but quite dramatically with mica concentration.
XRD is used to identify and roughly quantify the amount of mica in aggregate. Spectra from samples of single-source fines, containing varying quantities of mica, clearly indicate the change in mica content. However, when a known quantity of mica is added to fines of different aggregate sources, the spectrum generated shows little in common with the previous samples, which makes it difficult to estimate the mica content. XRD analysis is most repeatable when crystals are randomly oriented in the sample. Because mica flakes tends to lie flat, they tend to be somewhat ordered in their orientation, particularly when the grains of material are much larger than 1 micron. Hence, while the XRD is a powerful tool for helping determine the presence of mica, its limitations are evident from this study.
Nanoindentation is used to determine the hardness and stiffness of mica-mastic at the micro scale. It is shown that mastic with no mica becomes much harder after long term aging, indicating embrittlement. Mastic with low concentrations of mica is shown to be only a little harder. Mastic with 10% mica becomes softer after long term aging.
The more traditional test of BBR is used to study the mica-mastic at a macro scale and varying temperature. The results of this experiment confirmed the unexpected results of the nanoindenter. Mastic with less than 5% mica in the fines behaves similarly to binder. However, at a mica content of 10%, the stiffness decreases after long term aging. This might suggest that mica reduces aging effects.